Sulfur oxides (SOx) such as sulfur dioxide are contained in discharge gases generated by thermal power stations, plants such as chemical-production plants, metal-processing plants, sintering plants, and paper-making plants, and gas provided with a boiler employing a fuel such as coal or heavy oil. Thus, a flue gas desulfurization apparatus is employed in order to remove SOx contained in discharge gases. The flue gas desulfurization apparatus removes SOx contained in a discharge gas, by causing SOx to be adsorbed by a porous carbon material such as activated carbon fiber, oxidizing a sulfur component by oxygen contained in the discharge gas in the presence of the porous carbon material serving as a catalyst, and absorbing the oxidation product in water, to thereby form sulfuric acid, which is removed from the porous carbon material.
Among conventional flue gas desulfurization apparatuses, some contain a catalyst unit formed of plate-like activated carbon fiber sheets and corrugated activated carbon fiber sheets, which are alternatingly juxtaposed. In such apparatuses, water is added dropwise to activated carbon fiber contained in the catalyst unit, and a discharge gas is caused to pass through conduits provided between the sheets, whereby a sulfur component is removed in the form of sulfuric acid. In order to enhance discharge gas purifying performance (desulfurization efficiency), water must be added to activated carbon fiber so that uniform water distribution is attained. In addition, in order to prevent an increase in the size of an auxiliary facility for supplying water, a minimum required amount of water must be evenly added to the activated carbon fiber.
In one method (known as a lime-gypsum method) for removing sulfur oxides by employment of a flue gas desulfurization apparatus, a sulfur component contained in a discharge gas is collected in gypsum form by use of limestone slurry or slaked lime slurry serving as an absorbent. In an alternative method called the dry adsorption method, activated carbon is used in dry format.
The aforementioned conventional lime-gypsum method includes spraying of limestone slurry or slaked lime slurry into a discharge gas, whereby humidifying and cooling of the discharge gas and absorption of SOx are performed simultaneously. Accordingly, a large amount of slurry must be circulated, thereby requiring power and a large amount of water for circulating the slurry. In addition, since the thus-formed gypsum is in slurry form, an apparatus for separating water from the slurry so as to collect gypsum is required. Thus, when the lime-gypsum method is employed, dimensions and complexity of the desulfurization facility unavoidably increase.
The dry adsorption method requires a large amount of heat for releasing an adsorbed sulfur component from activated carbon through heating. In addition, there arise problems such as disposal of the formed dilute sulfuric acid and loss of the employed adsorbent. Therefore, demand has arisen for a desulfurization apparatus which can produce sulfuric acid during desulfurization without requiring an absorbent for sulfur oxides or a large desulfurization facility.
In this connection, there has been proposed an apparatus for removing SOx contained in a discharge gas in which SOx contained in the discharge gas is adsorbed by a porous carbon material such as activated carbon fiber, a sulfur component is oxidized by oxygen contained in the discharge gas in the presence of the porous carbon material serving as a catalyst, and the oxidation product is absorbed in water, to thereby form sulfuric acid, which is removed from the porous carbon material (see Japanese Patent Application Laid-Open (kokai) No. 11-347350).
The above conventional flue gas desulfurization apparatus employing activated carbon fiber includes an activated carbon fiber board, disposed in an adsorption tower, for adsorbing SOx contained in a discharge gas. In the desulfurization apparatus, a discharge gas is fed from the bottom of the tower, and SO2 is oxidized on the surface of the activated carbon fiber, to thereby form SO3. The thus-formed SO3 is reacted with supplied water, to thereby form sulfuric acid (H2SO4).
A considerable amount of discharge gas is generated from a boiler combusting a fuel such as coal or heavy oil. Such a large amount of discharge gas must be treated continuously to thereby enhance desulfurization efficiency. In order to perform continuous operation, it is essential that a large adsorption tower be employed. However, there is desired an adsorption tower which attains higher desulfurization efficiency of activated carbon fiber with a desulfurization system of small size.
In order to effectively attain catalytic action, catalytic reaction conditions must be optimized, and SO3 produced through oxidation of SO2 contained in a discharge gas must be effectively removed by use of water. In addition, in order to avoid increase in size of an auxiliary water-supply facility, water must be distributed uniformly in the catalyst through addition of a minimum required amount of water.
When industrial water or similar water is supplied to the catalyst, the total cost of the system increases. Therefore, improvement in efficiency of the system is required.
In this connection, utilization of waste water produced from the system or similar water is a conceivable approach for solving the problem. However, when this approach is actually employed, catalyst activity problematically lowers.
Since water is needed for catalytic action, the catalyst must maintain an appropriate water content. When the catalyst is not sufficiently wet, the catalytic action cannot fully be attained, which is problematic. Particularly at the time of starting the desulfurization apparatus, poor wetting conditions are problematic.
When the aforementioned plants are stopped, water for humidifying-cooling and additional water are fed to a desulfurization apparatus, since hot airflow is continuously fed to the apparatus even after the boilers are stopped. In this case, since a discharge gas containing sulfur oxides is not fed to the desulfurization apparatus, the concentration of the formed sulfuric acid gradually lowers. When sulfuric acid having a concentration below a certain level is used to form gypsum, separation and collection of the product become difficult, which is problematic. Thus, conventionally, such low-concentration sulfuric acid which cannot be used to produce gypsum must be treated as industrial waste, which is also problematic.
In the case where dilute sulfuric acid is produced without producing gypsum, when the concentration of dilute sulfuric acid is excessively low, the size of the concentration facility must be increased, thereby problematically elevating the cost of the sulfuric acid production facility.
The present invention has been accomplished under the above-described circumstances. Thus, an object of the present invention is to provide a flue gas desulfurization apparatus comprising a catalyst unit formed of at least one activated carbon fiber board allowing uniform water distribution therein.
The present invention has been accomplished under the above-described circumstances. Thus, another object of the present invention is to provide a desulfurization method for removing sulfur oxides (SOx) by evenly adding water to an activated carbon fiber board.
The present invention has been accomplished under the above-described circumstances. Thus, still another object of the present invention is to provide a desulfurization method which for removing sulfur oxides (SOx) by evenly adding a minimum required amount of water to an activated carbon fiber board.
The present invention has been accomplished under the above-described circumstances. Thus, still another object of the present invention is to provide a flue gas desulfurization apparatus which discharges no industrial waste and which attains high efficiency.
The present invention has been accomplished under the above-described circumstances. Thus, still another object of the present invention is to provide a flue gas desulfurization apparatus which requires no absorbent for sulfur oxides, can be operated without a large desulfurization facility, and can produce high-concentration sulfuric acid during desulfurization; i.e., to provide a flue gas desulfurization apparatus which can reduce the amount of supplied water and which attains uniform water distribution.
The present invention has been accomplished under the above-described circumstances. Thus, still another object of the present invention is to provide a flue gas desulfurization apparatus which can perform desulfurization reaction at high efficiency by use of activated carbon fiber, which provides a simple desulfurization system, and which attains high efficiency and small size.
The present invention has been accomplished under the above-described circumstances. Thus, still another object of the present invention is to provide a flue gas desulfurization apparatus which assures high overall efficiency of a desulfurization system and which maintains desulfurization performance over a long period of time.